Hydraulic Control Valve Brake System

ABSTRACT

A hydraulic control valve for an ATV or motorcycle takes hydraulic input from a hand brake lever as an input to a first cavity of the control valve with a movable piston therein, and from a foot brake lever as an input to a second cavity of the control valve with a movable piston therein, with the two pistons being linked. A bypass channel extends around the piston in the first cavity, which is closed off when the first piston moves longitudinally. The output of the first cavity hydraulically controls braking of the front wheel(s). The foot brake lever also pressurizes a direct line to brake the rear wheel(s) that doesn&#39;t go through the control valve.

CROSS-REFERENCE TO RELATED U.S. APPLICATION(S)

None.

FIELD OF THE INVENTION

The present invention relates to hydraulic control valves in brakesystems with independent braking of front and rear wheels, primarily foruse in handlebarred, straddle-type power vehicles such as motorcyclesand all-terrain vehicles (“ATVs”).

BACKGROUND OF THE INVENTION

Many vehicles include a braking system in which the brakes are actuatedthrough pressure in hydraulic fluid lines, controlled by movement of abraking lever. The brakes typically include a braking caliper operatingin frictional engagement with a rotating disc for each wheel beingbraked. When the braking lever is moved, it drives a braking pump orotherwise pressurizes the hydraulic oil in the system, pushing thepad(s) of the braking caliper against the disc. Hydraulic disc brakeshave been widely used on motorcycles, ATVs and other vehicles. Thehydraulic braking system needs to be flexible and reliable, and simpleto operate. The hydraulic braking system should also be relativelyinexpensive to manufacture, should allow for ease of maintenance, andneeds to be robust for long life over years of outdoor use in variousweather conditions.

Motorcycles and ATVs are straddle vehicles, which differ from cars byhaving a higher center of gravity, as well as usually having a shorterwheel base. When the straddle vehicle is braked, the rider's weighttends to drift forward significantly, placing more weight on the frontwheel(s) than the rear wheel(s). The changing center of gravity can beworse for novice drivers than for expert drivers, particularly duringunexpected braking to avoid a collision. More braking force may beneeded on the front wheel to slow the front wheel at an equivalent rateto the rear wheel, particularly during heavy braking. To providedifferent braking forces on the front wheel(s) than on the rearwheel(s), some motorcycles and ATVs have front wheel braking systemswhich operate independently of the rear wheel braking system. Skilleddrivers will actuate the front braking device and the rear brakingdevice appropriately according to the road and weather conditions anddesired braking deceleration, allowing the vehicle to be braked stablyand reliably. However, for unskilled drivers, the traditionalindependently-operated front and rear brakes can result in potentiallyunsafe operation. Overbraking the front wheel can cause the front wheelto lock, reducing steering ability and potentially causing rolloveraccidents. Overbraking the rear wheel reduces the braking efficiency andcan cause the rear wheel to lock, potentially causing sideslipaccidents. Better braking systems are needed, particularly directed atuse in handlebarred, straddle power vehicles such as motorcycles andATVs.

BRIEF SUMMARY OF THE INVENTION

The present invention is a hydraulic control valve for a vehicle brakingsystem, and a vehicle braking system using the hydraulic control valve.The hydraulic control valve takes hydraulic input from two sources, suchas a hand brake lever and a foot brake lever, in a way which allowseither the hand brake lever or the foot brake lever to brake one of thewheels, typically the front wheel(s). The other wheel(s), typically therear wheel(s), is separately braked by only one of the hand brake leveror the foot brake lever, via a direct line that doesn't go through thecontrol valve. In one aspect, the hydraulic line for the hand brakelever is provided as an input to a first cavity of the control valvewith a movable piston therein, and the hydraulic line for the foot brakelever is provided as an input to a second cavity of the control valvewith a movable piston therein, with the two pistons being linked. Abypass channel extends around the piston in the first cavity, whichbypass channel is closed off when the first piston moves longitudinally.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the layout of a braking system inaccordance with a first embodiment of the present invention;

FIG. 2 is a top plan view of the preferred hydraulic control valve usedin the braking system of FIG. 1.

FIG. 3 is a side elevational view of the preferred hydraulic controlvalve of FIG. 2;

FIG. 4 is an end view of the preferred hydraulic control valve of FIGS.2 and 3;

FIG. 5 is a cross-sectional plan view of the preferred hydraulic controlvalve of FIGS. 2-4 shown in a first normal rest position;

FIG. 6 is a cross-sectional plan view of the preferred hydraulic controlvalve of FIGS. 2-5 shown in a second position during operation of thefoot brake lever;

In these drawings, the reference numerals are as follows:

braking system 10;

hand brake lever 12;

handlebars 14;

hand grip 16;

foot brake lever 18;

braking fluid reservoir 20;

calipers 22;

discs 24;

control valve 26;

front input port 28;

front output port 30;

rear input port 32;

hand brake line 34;

connector 36;

rear input line 38;

rear direct line 40;

valve body 42

front piston 44;

front cavity 46;

rear piston 48;

rear cavity 50;

piston rod 52;

front piston seal 54;

rear piston seal 56;

bypass channel 58;

rear bleed port 60;

divider wall 62;

press rings 64;

seals 66;

front cavity compression spring 68;

rear cavity compression spring 70;

front end plug 72;

rear end plug 74;

front plug O-ring 76;

rear plug O-ring 78;

bleed screw 80;

bleed screw cap 82;

bypass channel machining port 84;

allen wrench plug 86;

mounting bolt holes 88; and

front brake line 90.

While the above-identified drawing figures set forth a preferredembodiments, other embodiments of the present invention are alsocontemplated, some of which are noted in the discussion. In all cases,this disclosure presents the illustrated embodiments of the presentinvention by way of representation and not limitation. Numerous otherminor modifications and embodiments can be devised by those skilled inthe art which fall within the scope and spirit of the principles of thisinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As is common in handlebarred, straddle-type power vehicles, a brakingsystem 10 in accordance with the present invention allows independent ordifferent braking of front wheel(s) relative to the rear wheel(s). Oneof the preferred inputs for the braking system 10 is a hand brake lever12, typically mounted on the steering handlebars 14 of the vehicleadjacent a hand grip 16. The other preferred input for the brakingsystem 10 is a foot brake lever 18. The braking system 10 operates onflow and pressure of a fluid such as hydraulic oil, and the brakingsystem 10 includes a braking fluid reservoir 20. When the hand brakelever 12 or the foot brake lever 18 are pressed, the braking fluidcauses the calipers 22 to frictionally engage the discs 24 for eitherthe front wheel(s), the rear wheel(s) or both. While FIG. 1 shows asystem 10 with two front wheels and two rear wheels such as in an ATV,the system 10 could alternatively have a single front wheel and a singlerear wheel such as in a motorcycle, operating under the same principles.While FIG. 1 shows a hand brake lever 12 on a handlebar 14 and a footbrake lever 18, the inputs could be different, such as two hand brakelevers or paddles or two foot brake levers or pedals, with the importantconsideration being that there are two separate inputs to the brakingsystem 10 controlled by the driver of the vehicle.

As will be explained, the arrangement, front/rear vehicle orientationand front/rear control valve orientation shown in FIG. 1 is consideredbest for most straddle vehicle operations, providing for an intuitiveand easily controlled adjustment of braking pressure on both the frontand rear wheels. Alternatively, the hand brake lever 12 and the footbrake lever 18 could be reversed, with the rest of the FIG. 1 schematicunchanged. As another alternative, the control valve 26 could be mountedside-to-side on the vehicle, vertically, or with the front/rearorientation reversed. As additional alternatives, either with the handbrake lever 12 and the foot brake lever 18 as shown or with the handbrake lever 12 and the foot brake lever 18 reversed, and with the valve26 mounted as shown or with a different mounting orientation, thefront/rear orientation of the braking system 10 on the vehicle could bereversed. Because the vehicle orientation and control valve orientationshown in FIG. 1 is considered best, the terms “front” and “rear” will beused in the specification in accordance with FIG. 1. However, it shouldbe noted that other mounting arrangements and orientations are withinthe scope of certain embodiments of the present invention.

The primary improvement of the present invention resides in a controlvalve 26 connected between the two levers 12, 18 and the brakes 22. Thecontrol valve 26 includes a front input port 28, a front output port 30and a rear input port 32. A liquid output port of the hand brake 12 isin fluid communication with the front input port 28 of the control valve26 through a hand brake line 34. A liquid output port of the foot brake18 is divided such as in a connector 36 into two lines, a first line 38to the rear input port 32 of the control valve 26, and a second directline 40 to the rear braking caliper(s) 22 r.

The internal operation of the control valve 26 is best understood withreference to the cross-sectional views of FIGS. 5 and 6. The controlvalve 26 includes a valve body 42, with a front piston 44 which canslide longitudinally in a front cavity 46 and a rear piston 48 which canslide longitudinally in a rear cavity 50. The front piston 44 and therear piston 48 are rigidly connected together through a piston rod 52,so the front piston 44 and the rear piston 48 move longitudinallytogether at the same speed and distance. Alternatively, a differentlinkage mechanism could be used, providing the rear piston 48 with amechanical advantage or otherwise making it so the rear piston 48 movesthe front piston 44 at a different speed or amount of movement as therear piston 48 moves. During operation, the front cavity 46 is filledwith braking fluid both forward and rearward of the front piston 44,whereas the rear cavity 50 is filled with braking fluid only forward ofthe rear piston 48. The front piston 44 includes a front piston seal 54to ensure no braking fluid flow between the front piston 44 and thevalve body 42 where the front piston 44 slides against the valve body42. The rear piston 48 includes a rear piston seal 56 to ensure no flowof either braking fluid or air around the rear piston 48 where the rearpiston 48 slides against the valve body 42.

A front bypass channel 58 is in the valve body 42 toward the front ofthe front cavity 46, extending around the front piston 44 when the frontpiston 44 is in the normal rest position shown in FIG. 5. However, whenthe front and rear pistons 44, 48 slide toward the position shown inFIG. 6, the front piston 44 closes off the front bypass channel 58,sealing off the portion of the front cavity 46 behind the front piston44 from the flow induced by the hand brake lever 12.

In the preferred embodiment, the valve body 42 is machined with a singlecentral longitudinal chamber from end to end, as well as the front inputport 28, the front output port 30, the rear input port 32, a rear bleedport 60 and the front bypass channel 58 on the sides of the valve body42. Other manufacturing methods and other port arrangements can beprovided to result in the same effect, but the assembly method discussedbelow assumes the preferred machining operations to form the valve body42.

A divider wall 62 with a central opening therethrough for the piston rod52 is secured within the longitudinal chamber, such as through pressrings 64. Washers and seals 66 are provided to ensure no braking fluidflow through the divider wall 62 during operation of the control valve26. After the divider wall 62 is in place, a front cavity compressionspring 68 is positioned in the front cavity 46, and then the frontpiston 44 and the rear piston 48 are rigidly secured together such asthrough rotational tightening of a threaded connection in the piston rod52, with the piston rod 52 extending through the divider wall 62 and thefront cavity spring 68. As will be described below, the cross-sectionalareas of the piston rod 52, both where it enters the divider wall 62from the rear cavity 50 and where it exits the divider wall 62 into thefront cavity 46 (which are depicted as being equal to each other, butcould alternatively differ from each other), are matters of designchoice to achieve the desired braking profile of the system 10.

A rear cavity compression spring 70 is positioned in the rear cavity 50.When the valve body 42 is machined from both ends, a front end plug 72and a rear end plug 74 can be used to close off the ends of the frontand rear cavities 46, 50. The front end plug 72 includes threads so itcan be rotationally tightened onto the valve body 42 to enclose thefront cavity 46. A front plug ring seal or O-ring 76 is provided toensure no leakage of braking fluid from the front cavity 46. The rearend plug 74 similarly includes threads so it can be rotationallytightened onto the valve body 42 to enclose the rear cavity 50. In thepreferred embodiment, a rear plug ring seal or O-ring 78 is provided toentrap a compressible gas such as air in the space behind the rearpiston 48 in the rear cavity 50. Alternatively, an air vent (not shown)could be provided in the rear side of the rear cavity 50 beyond thethrow of the rear piston 48.

While the various seals 54, 56, 66. 76, 78 of the preferred embodimentare quite effective in controlling the braking fluid, some leakage offluids may occur, particularly air migrating past the rear piston seal56. Any air or other compressible fluid in the hydraulic system 10 tendsto make the brakes sluggish and the performance spongy. In theembodiment shown, the rear bleed port 60 is provided and closed off witha bleed screw 80. The bleed screw 80 provides a primary bleedingmechanism substantially as known in bleeding hydraulic brake systems. Ableed screw cap 82 may snap on or otherwise be attached on theprojecting end of the bleed screw 80. The bleed screw cap 82 ispreferably attached to the bleed screw 80 such as through a flexibletie, thereby keeping the bleed screw cap 82 from being dropped when theuser bleeds the rear portion of the brake system 10. For low costmanufacture and ease of use, the bleed screw cap 82 may be molded of aflexible plastic or rubber material.

In the embodiment shown, a bypass channel machining port 84 is closedoff such as by an allen wrench set screw plug 86. If desired, the plug86 can be loosened to provide a secondary bleeding mechanism for thecontrol valve 26 and brake system 10, for bleeding the front portion ofthe brake system 10.

The valve body 42 preferably includes one or more mounting bolt holes88, for receiving mounting bolts (not shown) in securing the controlvalve 26 relative to the vehicle. Any other convenient mounting systemcould alternatively be used.

How the control valve 26 operates to control the braking operation ofthe vehicle depends on how the lever controls 12, 18 are manipulated.The easiest operating mode to understand is when the hand brake lever 12is gently pressed without pressing the foot brake 18. When the handbrake 12 is operated, hydraulic oil enters into the front cavity 46through the front input port 28, flows through the bypass channel 58 andflows towards the front braking caliper 22 through the front output port30, only braking the front wheels. As long as the applied pressureremains reasonable (i.e., gentle non-vigorous hand braking of the frontwheel(s)), the front and rear pistons 44, 48 tend to be held in theirrest position shown in FIG. 5, dominated by the spring force of thecompression springs 68, 70. The braking pressure on the front wheelbrake calipers 22 f is identically controlled by the gentle brakingpressure applied by the hand lever 12. With the braking fluid beingsubstantially incompressible, the distance the front wheel calipers 22 fmove is identically controlled by the flow of brake fluid out of or backinto the hand lever 12, proportional to the ratio of fluid area of thefront brake lever relative to the fluid area of the front brake caliper22 f. The ratio of fluid area of the front brake lever 12 relative tothe fluid area of the front brake caliper 22 f can be selected as amatter of design choice of the system 10, to provide the desiredmechanical advantage and travel distance required of the front brakelever 12 as known in the art. If desired, a brake fluid pump (not shown)can be provided to increase the mechanical advantage of the system 10.

A different situation occurs whenever the foot brake lever 18 is pressed(with or without pressing the hand brake lever 12), and can also occurif the hand brake lever 12 is pressed vigorously even without pressingthe foot brake 18. The front and rear pistons 44, 48 move when theforces pressing the pistons 44, 48 rearward exceed the forces pressingthe pistons 44, 48 forward. The forces pressing the pistons 44, 48rearward are: a) the pressure force of braking fluid on the front faceof the front piston 44, multiplied by its area (in the embodiment shown,the full cross-sectional area of the bore); and b) the pressure force ofbraking fluid on the front face of the rear piston 48, multiplied by itsarea (in the embodiment shown, the cross-sectional area of the boreminus the cross-sectional area of the piston rod 52 where it enters thedivider wall 62). The forces pressing the pistons 44, 48 forward are c)the pressure force of braking fluid on the rear face of the front piston44, multiplied by its area (in the embodiment shown, the cross-sectionalarea of the bore minus the cross-sectional area of the piston rod 52where it exits the divider wall 62); d) the pressure force of thecompressible air on the rear face of the rear piston 48, multiplied byits area (in the embodiment shown, the cross-sectional area of thebore); e) the spring force of two compression springs 68, 70; and f) themomentum force needed to decelerate the mass of the front and rearpistons 44, 48 and piston rod 52 during braking (decelerating thepistons 44, 48 at the rate that the vehicle is decelerating).

The spring constants and the area differential between front and rearfaces of the rear piston 48 are selected so only a minimum of rear wheelbraking can occur using the foot lever 18 without also moving thepistons 44, 48 and engaging the front brakes 22 f as well. During mostoperation of the vehicle, the front brake 22 f can be gently braked byitself using the hand lever 12, or both front and rear wheels 22 f, 22 rcan be braked using the foot lever 18, alone or in combination with thehand lever 12.

Once the front piston 44 moves even a slight distance, the front piston44 closes off the bypass channel 58, trapping the amount of brake fluidwhich is rearward of the front piston 44 in the front cavity 46 and inthe brake line 90 to the front calipers 22 f. After this close off ofthe bypass channel 58, the control valve 26 causes the braking system 10to operate in a different way than before.

After closing off of the bypass channel 58, increasing brake pressure byfurther moving the foot brake lever 18 not only increases pressure onthe rear calipers 22, but also moves the pistons 44, 48 further rearwardby causing flow into the rear cavity 50 of the control valve 26. Therearward movement of the front piston 44 increases pressure to the frontbrake calipers 22 while simultaneously decreasing response pressure onthe hand brake lever 12. With hands that are typically more sensitivethan feet, most drivers feel the decreasing response pressure on thehand brake lever 12, and responsively tend to further move the handbrake lever 12 at the same time as they push on the foot brake lever 18.Thus, drivers that can simultaneously operate foot and hand brakes sothe braking forces are appropriately divided between the front and rearwheels based on their skill level.

After closing off of the bypass channel 58, increasing brake pressure byfurther moving the hand brake lever 12 similarly moves the pistons 44,48 further rearward; while rearward movement of the front piston 44increases pressure to the front brake calipers 22, rearward movement ofthe rear piston 48 also decreases response pressure on the foot brakelever 18 and decreases any pressure to the rear brake calipers 22. Ifsimultaneously using both the hand and foot levers 12, 18, novicedrivers who are “handsy” are less likely to feel and respond to thedecreasing foot response pressure and less likely to appropriatelybalance between hand and foot brake pressure. Such novice drivers 13 whoare also less likely to control their center of gravity duringbraking—end up applying more brake pressure to the front wheel(s) 22 f,as required to brake the front wheel(s) since the front wheel(s) supportmore of such riders' weight.

At the same time, closing off of the bypass channel 58 changes theresponse flow rate of the hand control 12. While the bypass channel 58was open, the brake fluid flow rate out of the hand brake lever 12 wasequal to the brake fluid flow rate into the front caliper 22 f. Afterthe bypass channel 58 closes, the brake fluid flow rate into the frontcaliper 22 f is the change in volume in the front cavity 46 behind thefront piston 44, which (due to the volume of the piston rod 52) is lessthan the change in volume in the front cavity 46 in front of the frontpiston 44. This increased travel rate of the hand brake lever 12 resultsin a lower likelihood of overbraking and locking up the front wheel.

Note that all of these forces a)-f) described above can be controlled bythe designer of the control valve 26. For instance, the relative areasof the front and back faces of each of the front and rear pistons 44, 48(as well as any mechanical advantage or disadvantage provided in thelinkage between the rear piston 48 and the front piston 44, if the frontand rear pistons 44, 48 don't move the same travel distance) can beselected as desired by the designer of the control valve 26 to achievethe desired balance between the hand brake 12 and the foot brake 18. Theinitial spring forces and spring constants of the springs 68, 70, aswell as how far the pistons 44, 48 must move before the front piston 44blocks the bypass channel 58, can be selected as desired by the designerof the control valve 26 to select how quickly and under what conditionsthe control valve 26 switches to close off the bypass channel 58. Thefront-back orientation of the control valve 26 can be changed, based onhow the control valve 26 is mounted on the vehicle, to control whetherdeceleration forces (primarily based on mass of the pistons 44, 48)assist or counteract movement of the pistons 44, 48, making theswitchover closing off of the bypass channel 58 more or less likelyduring hard braking. In the most preferred embodiment, the diameter ofthe bore in the valve body 42 is 19 mm, and the diameter of the pistonrod 52 is 6 mm, both being cylindrical. This causes the front piston 44to have a front face area of about 284 mm² and a rear face active areaof about 255 mm², and causes the rear piston 48 to have a front facearea of about 255 mm² and a rear face active area of about 284 mm². Inthe most preferred embodiment, the front cavity spring 68 has a springconstant of 5.9 kN/m, and the rear cavity spring 70 has a springconstant of 14.1 kN/m. In the most preferred embodiment, a completethrow of the foot lever 18 followed by a complete throw of the handlever 12 moves the pistons 44, 48 a distance of about 5 to 7 mm, suchthat the volume of air behind the rear piston 48 is compressed 56 to 78%during maximum braking.

Note also that the order of braking between the hand lever 12 and thefoot lever 18 affects the position of the hand lever 12 for brakingengagement. If the hand lever 12 is gradually engaged first, the handlever 12 can be significantly pressed before the pistons 44, 48 move andthe bypass channel 58 is closed off. The high pressure response of thehand lever 12 will then be at the end of its throw after the foot lever18 is engaged, with a first portion of the throw of the hand lever 12being pressureless and inactive until the driver releases the foot lever18. If the foot lever 18 is engaged first in a significant way, thepistons 44, 48 will move and the bypass channel 58 will be closed offbefore the hand lever 12 has moved at all. Then the hand lever 12 willstill be fully active contributing to braking pressure throughout itsthrow until the driver releases the foot lever 18, rather than having afirst portion of the hand brake throw which is pressureless.

In the preferred embodiment, when the driver releases pressure on boththe hand lever 12 and the foot lever 18, the control valve 26 is againdominated by the compression springs 68, 70 and the air pressure on therearward side of the rear piston 48, pushing the pistons 44, 48 back tothe position shown in FIG. 5. The combination between the compressionsprings 68, 70 and the air pressure can be used by the designer of thecontrol valve 26 to result in the desired resetting force curve. Forinstance, if the air side of the rear cavity 50 is vented to atmosphere,then the spring constants can be selected to provide a linear resettingforce on the pistons 44, 48. If the compression springs 68, 70 areomitted, then it is just the air pressure on the rearward side of therear piston 48 which resets the pistons 44, 48 back to the positionshown in FIG. 5. The value of this air pressure depends upon therelative change in volume of the rear cavity 50 to the rear of the rearpiston 48 which was caused when the pistons 44, 48 moved away from therest position shown in FIG. 5. For instance, by designing the rearcavity in one shape, an 8 mm movement of the rear piston 48 could reducethis air volume by 50% (doubling the air pressure force for resettingthe pistons 44, 48), while a 4 mm movement of the rear piston 48 couldreduce this air volume by 25% (increasing the air pressure force forresetting the pistons 44, 48 by 33%). By designing the rear cavity in adifferent shape, an 8 mm movement of the rear piston 48 could reducethis air volume by 95% (increasing the air pressure force for resettingthe pistons 44, 48 by a factor of 20), while a 4 mm movement of the rearpiston 48 could reduce this air volume by 47.5% (increasing the pressureforce for resetting the piston by 2.1 times). Because the increase inair pressure can have a geometric effect on the force resetting thepistons 44, 48 while the springs 68, 70 have a linear effect on theforce for resetting the pistons 44, 48, any of a wide range of differentcurves of resetting force can be selected by the control valve 26designer. While compression springs 68, 70 are shown, other types ofsprings such as disc springs or torsion springs could alternatively beused to place a resetting force on the pistons 44, 48. A dashpot (notshown) could also be added to adjust the rate of resetting forceapplication.

It will also be noted that the present invention provides andincorporates two independent sets of brake oil lines, i.e. lines 34 and90 are completely separated from lines 38 and 40. If a leak occurssomewhere in the system 10, it will only affect one portion of thesystem 10, i.e., any single leak cannot completely disable both thefront and rear brakes 22 f, 22 r. By its use of two independent sets ofbrake oil lines, including the dividing wall 62 in the control valve 26to achieve such separation, the present invention improves bothreliability and security of the whole braking system 10, ensuring thesafety of the driver.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A hydraulic control valve for a vehicle braking system, the controlvalve comprising: a valve body defining a first brake fluid cavity; afirst piston which is longitudinally moveable within the first brakefluid cavity; a first input port in the valve body for brake fluid influid communication with a first area of the first piston; a firstoutput port in the valve body for brake fluid in fluid communicationwith a second area of the first piston; a bypass channel for the firstbrake fluid cavity, the bypass channel allowing brake fluid flow aroundthe first piston when the first piston is in a first longitudinalposition, the bypass channel being closed off when the first piston isin a second longitudinal position; and a linkage for selectively movingthe first piston from the first longitudinal position to the secondlongitudinal position.
 2. The hydraulic control valve of claim 1,wherein the valve body defines a second cavity, wherein the controlvalve comprises: a second piston with is longitudinally moveable withinthe second cavity; and a second input port in the valve body for brakefluid in fluid communication with a first area of the second piston; andwherein the linkage comprises: a piston rod extending from the firstpiston through a divider wall to the second piston, such that the firstand second pistons move simultaneously in a longitudinal direction whenbrake fluid introduced to the second cavity through the second inputport acts on the first area of the second piston.
 3. The hydrauliccontrol valve of claim 2, further comprising: an enclosure in the valvebody for holding a compressible fluid acting on a second area of thesecond piston, with compressible fluid pressure biasing the first andsecond pistons longitudinally toward the first longitudinal position. 4.The hydraulic control valve of claim 3, further comprising a bleed screwfor bleeding the second cavity of compressible fluid.
 5. The hydrauliccontrol valve of claim 2, wherein the piston rod has a cross-sectionalarea where the piston rod enters the divider wall which is equal to across-sectional area where the piston rod exits the divider wall.
 6. Thehydraulic control valve of claim 2, wherein the piston rod comprisesscrew threads to rigidly attach the first piston relative to the secondpiston.
 7. The hydraulic control valve of claim 2, further comprising aspring biasing the first and second pistons longitudinally toward thefirst longitudinal position.
 8. The hydraulic control valve of claim 2,wherein the first area of the first piston is larger than the first areaof the second piston.
 9. The hydraulic control valve of claim 2, whereinthe second area of the first piston is equal to the first area of thesecond piston.
 10. The hydraulic control valve of claim 2, wherein thefirst and second cavities are collinearly arranged on the same centerline through the valve body, and a formed as a single through hole inthe valve body, closed off by one or more plugs threadably received onthe valve body.
 11. The hydraulic control valve of claim 2, wherein thevalve body has a first end and a second end defining the longitudinaldirection, and sides extending from the first end to the second end,wherein the first input port, the first output port and the second inputport are disposed on the sides of the valve body.
 12. The hydrauliccontrol valve of claim 1, further comprising a spring biasing the firstpiston longitudinally toward the first longitudinal position.
 13. Thehydraulic control valve of claim 1, wherein the bypass channel isdefined in the valve body, and further comprising a machining port inthe valve body for machining the bypass channel, the machining portbeing closed off by a removable plug.
 14. A brake system for a vehicle,comprising: a first braking control mountable on the vehicle for controlby a driver, the first braking control pressurizing brake fluid; asecond braking control mountable on the vehicle for separate control bythe driver, the second braking control pressurizing brake fluid, a brakeon a first wheel of the vehicle being in fluid communication with brakefluid pressurized by the second braking control, so the brake on thefirst wheel is solely controlled by the second braking control; acontrol valve mounted in the vehicle, the control valve comprising: avalve body mounted on the vehicle; a first input port in the valve body,the first input port receiving brake fluid pressurized by the firstbraking control; a second input port in the valve body, the second inputport receiving brake fluid pressurized by the second braking control;and a first output port in the valve body, the first output port outputbeing in fluid communication with a brake on a second wheel of thevehicle, wherein the first output port can be pressurized either throughpressurization from the first braking control or pressurization from thefirst braking control.
 15. The brake system of claim 14, wherein thecontrol valve further comprises: a first brake fluid cavity defined inthe valve body; a first piston which is longitudinally moveable withinthe first brake fluid cavity, with brake fluid through the first inputport being in fluid communication with a first area of the first piston;a second brake fluid cavity defined in the valve body; and a secondpiston which is longitudinally moveable within the second brake fluidcavity, with brake fluid through the second input port being in fluidcommunication with a first area of the second piston, the second pistonbeing linked to the first piston.
 16. The brake system of claim 15,wherein the control valve further comprises: a bypass channel for thefirst brake fluid cavity, the bypass channel allowing brake fluid flowaround the first piston when the first piston is in a first longitudinalposition, the bypass channel being closed off when the first piston isin a second longitudinal position.
 17. The brake system of claim 15,further comprising a resetting mechanism biasing the first piston towardthe first longitudinal position.
 18. The brake system of claim 14,wherein the first wheel is a rear wheel of the vehicle and the secondwheel is a front wheel of the vehicle.
 19. The brake system of claim 14,wherein the first braking control is a hand brake lever and the secondbraking control is a foot brake lever.
 20. The brake system of claim 14,wherein the brake fluid pressure/zed by the first braking control iscompletely separated from the brake fluid pressurized by the secondbraking control.